Final answer:
The verb 'fue' should be used to fill in the blank in the sentence.
Step-by-step explanation:
The correct verb to fill in the blank in the sentence 'un mes que fuimos al cine' is 'fue.' The sentence translates to 'it has been a month since we went to the movies.' In this case, 'fue' is the past tense conjugation of the verb 'ser' (to be), which is used to indicate the passage of time. The verb 'hacer' (to do/make) in option A is incorrect because it does not convey the intended meaning of the sentence.
Learn more about Spanish verb conjugation